Dynamic random access memories at present utilize what is known as a one transistor (1-T) cell to store bits of data. The 1-T cell has the advantage of low parts count, thus producing high density arrays. These cells are typically fabricated in a planar construction utilizing a single capacitor as a storage element and associated with some type of adjacent gate structure which normally is comprised of an MOS transistor. The area of the cell is therefore defined as the area of the transistor, the area of the capacitor and the area of the field isolation between cells.
When the 1-T cell originally came into use, it was fabricated in a planar structure utilizing a diffused region as both the lower plate of the capacitor and the drain of the transistor. The upper plate of the capacitor was fabricated from an upper metal layer separated from the diffused region by a capacitive oxide layer. As photolithographic techniques improved, densities increased without deviating from the planar structure. Although the relative size of the capacitor did decrease somewhat, techniques such as the Hi-C cell enabled the capacity of the capacitor to be increased while the surface area decreased. However, there are limitations to the density that can be achieved with a planar construction due to the minimum size of the transistor, soft error problems, etc.
There have been a number of capacitor construction techniques proposed to overcome the limitations of the planar construction such as trench capacitors and stacked capacitors. Trench capacitors utilize trenches in the substrate in which the capacitors are formed to provide an increased surface area for the capacitor. Stacked capacitors place the capacitor over the transistor to increase the surface area available for the cell.
To date, the only technique that has been utilized with any success in high density DRAMs is the trench capacitor cell. However, this procedure requires not only fabrication of repeatable trenches, but also formation of the capacitor oxide and upper plate of the capacitor within a trench. At present, no realizable DRAMs are being produced that allow for fabrication of the memory cell capacitor above the substrate.